Ultrasonic sensor systems may use a transmitter to generate and send an ultrasonic wave through a transmissive medium and towards an object to be detected and/or imaged. The ultrasonic transmitter may be operatively coupled with an ultrasonic sensor array configured to detect portions of the ultrasonic wave that are reflected from the object. At each material interface encountered by the ultrasonic pulse, a portion of the ultrasonic pulse may be reflected. In some implementations, an ultrasonic pulse may be produced by starting and stopping the transmitter during a short interval of time (e.g. less than 1 microsecond). An ultrasonic sensor system may include biometric sensors, such as fingerprint or handprint sensors, and/or other ultrasonic imaging applications.
Piezoelectric ultrasonic transducers are attractive candidates for such applications and may include piezoelectric micromechanical ultrasonic transducers (PMUTs) configured as a multilayer stack that includes a piezoelectric layer stack. The piezoelectric layer stack may include a layer of piezoelectric material such as, for example, a layer of polyvinylidene fluoride (PVDF) or a PVDF copolymer. The piezoelectric layer may convert vibrations caused by ultrasonic reflections into electrical output signals. In some implementations, the ultrasonic sensor system further includes a thin-film transistor (TFT) layer that may include an array of sensor pixel circuits that may, for example, amplify electrical output signals generated by the piezoelectric layer.
In some applications, a two-dimensional array of a large number of PMUT elements (a “PMUT array”) may be contemplated. For example an array of 1-5 million PMUTs may be contemplated for some large area ultrasonic sensors. In the absence of the presently disclosed techniques, the TFT layer of such a large area ultrasonic sensors may limit the current to the pixel elements and degrade transmission of the signals generated by the piezoelectric layer, due to narrow pixel to pixel address and signal traces.
As a result, improved PMUT drive/readout schemes are desirable, particularly for large PMUT arrays.